(a) Field of the Invention
The present invention relates to a plate type evaporator having a novel heat transfer surface construction, and more particularly it relates to a collision jet type plate evaporator.
(b) Description of the Prior Art
The plate type evaporator is designed so that the liquid to be evaporated is heat-exchanged with the heating medium through a heat transfer plate and thereby evaporated, with the transfer of heat effected while the liquid is boiling on the heat transfer surface.
When boiling starts inside a saturated liquid, vapor is generated with the particles of dust and air contained in the liquid serving as nuclei. When the so-called pool boiling inside a stationary liquid is considered, it is seen that bubbles of vapor are continuously generated until the temperature difference between the heat transfer surface and the saturated liquid reaches a certain value. The points of generation of bubbles of vapor on the heat transfer surface are called the boiling nuclei, and such boiling state is called nucleate boiling. Concerning the generation of bubbles of vapor, it is known that very small recesses on the heat transfer surface which act to suck the air contained in the liquid serve as cavities for the formation of nuclei, and that, therefore, satisfactory boiling heat transfer could be achieved by deliberately forming very small cavities on the heat transfer surface.
Processing the heat transfer surface for such purpose on an industrial scale, however, would be difficult. Therefore, a collision jet type plate evaporator in which a liquid to be evaporated is jetted onto the heat transfer surface to disturb the liquid on the heat transfer surface so as to promote boiling heat transfer, has already been proposed by me (e.g., U.S. Patent application Ser. No. 868,749). This comprises heat transfer plates, and jet plates formed with small holes, the arrangement being such that a first fluid, e.g., a liquid to be evaporated, is jetted through the small holes of the jet plates to collide with heat transfer surfaces of the heat transfer plates opposed thereto for heat exchange with a second fluid, e.g., a heating medium, flowing along the rear surfaces of said heat transfer plates, whereby the liquid is evaporated.
In the collision jet type, however, in the case of a flat plate, bubbles can hardly be generated in the vicinity of the points of collision of jets because of the successive supply of supercooled liquid to be evaporated. Even if they are generated, the disturbing action of the jets produces a kind of bubbling phenomenon, impeding the flow of the liquid or the bubbles remain unseparated from the liquid, lowering the heat transfer rate, only to form a losing factor in the case of nucleate boiling.